You are not registered yet. Please click here to register!


 
 
plc storereviewsdownloads
This board is for PLC Related Q&A ONLY. Please DON'T use it for advertising, etc.
 
Try our online PLC Simulator- FREE.  Click here now to try it.

---------->>>>>Get FREE PLC Programming Tips

New Here? Please read this important info!!!


Go Back   PLCS.net - Interactive Q & A > PLCS.net - Interactive Q & A > LIVE PLC Questions And Answers

PLC training tools sale

Reply
 
Thread Tools Display Modes
Old October 6th, 2006, 08:44 AM   #1
rta53
Lifetime Supporting Member
United States

rta53 is offline
 
rta53's Avatar
 
Join Date: Feb 2003
Location: North Carolina
Posts: 600
OT Limiting current output of VFD

We use VFDs, mainly AB Powerflex, to control pumps in our water purification systems. We try to maintain a certain pressure using a pressure transmitter as our reference. If I want the pump to have a minimum speed then I set the minimum hertz. If I want to limit the maximum speed of the pump I set a maximum hertz. We have an engineer who keeps asking me if there is a way to limit the amps that the VFD outputs. He says he is concerned about overloading the motor. I think he worries too much but he keeps bugging me about this. Can this be done?
  Reply With Quote
Old October 6th, 2006, 09:11 AM   #2
brucechase
Member
United States

brucechase is offline
 
brucechase's Avatar
 
Join Date: Sep 2004
Location: Augusta GA
Posts: 717
See parameter A089. But explain to the engineer that the drive has overload protection and the the current is based on the load that the motor "sees".
  Reply With Quote
Old October 6th, 2006, 09:28 AM   #3
Tom Jenkins
Lifetime Supporting Member
United States

Tom Jenkins is offline
 
Tom Jenkins's Avatar
 
Join Date: Apr 2002
Location: Milwaukee, WI
Posts: 5,254
Almost all VFDs have current limiting. One of the parameters available is full load amps of the motor. The details vary, and I haven't done an A-B drive for a while, but in general the drive will "clamp" the speed when current output reaches the max set for the motor. This is in addition to the overload protection Bruce mentioned.
  Reply With Quote
Old October 6th, 2006, 09:56 AM   #4
DJM
Member
United States

DJM is offline
 
Join Date: Aug 2006
Location: Georgia
Posts: 111
Quote:
Originally Posted by brucechase
See parameter A089. But explain to the engineer that the drive has overload protection and the the current is based on the load that the motor "sees".
Its interesting that the engineer is concerned about the current as if the drive is somehow forcing the current ON the motor vs the reality stated above. He must be an ME.

My ME acts like we techs are the last practitioners of some "Black art" or witchcraft. I can't tell you how many times he's suggested to me that my program may have gone bad; although this can happen, they don't usually go a 'little bit' bad!
  Reply With Quote
Old October 6th, 2006, 10:20 AM   #5
rta53
Lifetime Supporting Member
United States

rta53 is offline
 
rta53's Avatar
 
Join Date: Feb 2003
Location: North Carolina
Posts: 600
Quote:
Originally Posted by DJM
Its interesting that the engineer is concerned about the current as if the drive is somehow forcing the current ON the motor vs the reality stated above. He must be an ME.
Yes he is an ME. He often will not accept something I tell him the first time I tell him. I have to keep repeating myself. He's a very smart and nice guy but I once told him that dealing with him was like dealing with a teenager. (He's in his 60s)


Quote:
Originally Posted by Tom Jenkins
Almost all VFDs have current limiting. One of the parameters available is full load amps of the motor. The details vary, and I haven't done an A-B drive for a while, but in general the drive will "clamp" the speed when current output reaches the max set for the motor. This is in addition to the overload protection Bruce mentioned.
So lets say the FLA of the motor is 18 amps. Normally I would program this into the motor parameters. Are you saying it would be feasible to use, say 15, instead of 18. Would this achieve what he wants?
  Reply With Quote
Old October 6th, 2006, 10:23 AM   #6
rta53
Lifetime Supporting Member
United States

rta53 is offline
 
rta53's Avatar
 
Join Date: Feb 2003
Location: North Carolina
Posts: 600
Quote:
Originally Posted by brucechase
See parameter A089. But explain to the engineer that the drive has overload protection and the the current is based on the load that the motor "sees".
For some reason I don't seem to have this parameter. This is a PowerFlex 70 drive. What is this parameter?
  Reply With Quote
Old October 6th, 2006, 10:32 AM   #7
Tom Jenkins
Lifetime Supporting Member
United States

Tom Jenkins is offline
 
Tom Jenkins's Avatar
 
Join Date: Apr 2002
Location: Milwaukee, WI
Posts: 5,254
Quote:
Originally Posted by rta53
So lets say the FLA of the motor is 18 amps. Normally I would program this into the motor parameters. Are you saying it would be feasible to use, say 15, instead of 18. Would this achieve what he wants?
Not knowing what he wants (assuming he does) I can't say.

If you configure the drive to limit the max motor current to 15 amps it will clamp the speed when the output current is at 15 amps. However, there is no reason to restrict the current output to less than motor FLA as long as the motor is not operating in an unusually hot location or has had ventilation blocked. The motor will give normal life if operated continuously at FLA (and if it is a 1.15 Service Factor motor it will run above FLA without problems). Most winding heating and damage occurs during motor starting. A VFD limits inrush and exends motor life for frequently cycled motors.

It is poor engineering and poor economics to size a motor above the required load, add a 1.15 SF, use a VFD, and then limit current on top of it. At some point in time you can get rediculous with saftey factors, not to mention having lousy efficiency on a lightly loaded motor!

Last edited by Tom Jenkins; October 6th, 2006 at 10:35 AM.
  Reply With Quote
Old October 6th, 2006, 11:27 AM   #8
brucechase
Member
United States

brucechase is offline
 
brucechase's Avatar
 
Join Date: Sep 2004
Location: Augusta GA
Posts: 717
Quote:
Originally Posted by rta53
For some reason I don't seem to have this parameter. This is a PowerFlex 70 drive. What is this parameter?
Look at 147 and 148. I haven't used a PF70 yet but I have on the PF4 and PF40.

Please tell the ME that this is not necessary. Not only everything Tom said, but Powerfactor is reduced on a lightly loaded motor. If he really has a hard time understanding, suggest he call AB and talk to a drive tech. They have been helpful in the past and might shed some light on this for him.
  Reply With Quote
Reply
Jump to Live PLC Question and Answer Forum

Bookmarks


Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)
 
Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Forum Jump

Similar Topics
Thread Thread Starter Forum Replies Last Post
Encoder output types Orn Jonsson LIVE PLC Questions And Answers 10 May 9th, 2007 04:25 PM
toggle logic in 'c' language naishadgosai LIVE PLC Questions And Answers 11 August 6th, 2005 08:53 PM
Increase the output current rating of an S7-200 Preeya LIVE PLC Questions And Answers 16 September 18th, 2004 08:05 AM
Monitoring motor current Tim LIVE PLC Questions And Answers 51 April 9th, 2003 06:53 PM
Sequential Logic to AB PLC Tze LIVE PLC Questions And Answers 7 January 4th, 2003 11:41 AM


All times are GMT -5. The time now is 08:18 PM.


.